Within the past few years, the steady increase in the price of titanium dioxide (TiO.sub.2) has created a demand for products capable of replacing all or at least a substantial percentage of the TiO.sub.2 needed to opacify paints and other coating compositions. In naturally white products, such as snow, sea foam and flower petals, the whiteness arises from the interaction of light with a multiplicity of interfaces and microvoids therein which randomly scatter incident light within the supporting medium, thus creating the appearance of whiteness. It is known that the introduction of microvoid-containing polymer particles into compositions used for paint, paper coatings and the like perform in a similar manner to cause significant increases in basic composition opacity.
Recently a number of approaches to incorporating such microvoid-containing additives into paint and paper coating formulations have been disclosed. (See "Air and Polymer Extended Paints," Chalmers et al., European Supplement to Polymer Paint Colour Journal, Oct. 5, 1983, pp. 94-102.) One of these is the use of solid and/or vesiculated (hollow) polyester/styrene copolymer beads having a size in the range of 3 to 50 microns and marketed under the tradename "Spindrift." A major use of these particles has been to formulate matte paints with good polish resistance. However, unpigmented beads act as windows in the film and are generally used only for highly saturated colors. For cost savings in terms of opacity, the vesiculated bead is most generally used. As sold commercially, these particles are formulated to contain both air and TiO.sub.2 within the interior. In use, the particles act to enhance TiO.sub.2 efficiency while maintaining film integrity and, with a suitable latex, can achieve very high pigment volume contents (pvc) when counting the particles as a pigment.
In a second approach, aggregates of fine particles are formed by a process of addition polymerization under conditions of high shear in an aqueous medium. This process produces irregular shaped particles (sold under the name "microblock") which the manufacturer claims will produce higher film strengths than do spherical beads. Unlike the vesiculated beads described above, internal pigmentation with TiO.sub.2 is not found to be effective and these particles are usually combined with external TiO.sub.2 and a film extender to produce high pvc paints which are very flat.
In a third approach, fine particle size non-film-forming polystyrene hollow "beads", in the range of 100 to 600 nm are used. These "beads" have the same order of magnitude size as those of the latex normally used in emulsion paints, with which they are typically blended in a 1:1 ratio. These particles are sold under the tradename of "Plastic Pigment". Careful matching of these particles with the coalescing and other paint constituents is necessary to achieve optimum performance.
Yet a fourth approach involves a suspension of a non-film-forming hollow styrene/acrylic copolymer beads (typically 38% by weight, 52% by volume). As paints containing these beads dry, water, which is lost from the interior voids, is replaced by air. The resulting empty "microvoids" act as light scattering sources, thus increasing the hiding power of the dried film with 4 parts, by volume, of this opaque polymer being approximately equal in hiding power to 1 part of TiO.sub.2. The commercially available material, marketed under the tradename "Ropaque", has a uniform particle size of between 400 to 500 nm. In addition to opacifying by scattering incident light, it has been postulated that these particles also contribute to hiding power by uniformly spacing the TiO.sub.2 particles and, thus, helping to prevent TiO.sub.2 clumping. Further, because these particles have less surface area than the corresponding volumes of TiO.sub.2, a reduction in binder demand may be possible. Articles appearing in the literature concerning these products indicate that a reduction of up to about 25% in the TiO.sub.2 content in a paint formulation is achievable.
The process for manufacturing these microvoid particles is described in U.S. Pat. Nos. 4,469,825 issued to Kowalski et al. and 4,594,363 issued to Blankenship et al. These disclose the production and use of water insoluble particles made by sequential emulsion polymerization to form a swellable "core" of polymeric acid, which is then at least partially encased with a "sheath" polymer permeable to ammonia or an organic base, such as an amine. Sheath permeability is necessary to allow the base to contact the core. In the disclosed polymer composition, swelling of the core will occur when the acidic polymer is neutralized to a pH of 6 or higher. It is reported that when such particles are so neutralized, they will swell up by about 90-100%, thus causing cracklike microvoids to form in the thus swollen cores. When a paint film containing these particles is allowed to dry, a significant degree of light scattering and, therefore, inherent opacity is reportedly achieved. In so doing, the particles serve as a supplement or replacement of part or all of the pigmentary medium or extenders that would otherwise be required to achieve equivalent hiding power.
In the present invention, I have found that equivalent or even superior degrees of hiding power can be achieved with a core/sheath polymeric structure which requires neither swelling nor the presence of discrete microvoids.